I thought an isotropic antenna is mythical. What is an isotropic probe?

As referred by the following, site: http://www.emctd.com/

It's possible to synthesize an approximation to an isotropic antenna that is close enough for most measurement purposes over a limited frequency range.
Three bolometers mutually at right angles is one solution that springs to mind. Obviously this will be very insensitive, but it is a three axis RF detector with broad bandwidth, and so fulfills most criteria for an isotropic receiving antenna. But then, it is not a reciprocal device, so cannot be used for transmission.
If you extend this concept to a set of three loops or patches, each having good cross-polar isolation in its own plane, and combine them in a 3-way combiner, you perhaps have a narrow band isotropic radiator and detector.
None of these ideas meets the true full-spectrum reciprocal well-matched antenna necessary to be an isotrpoic radiator. To this extent therefore, the true isotropic radiator is a theoretical concept.

Isotropic antenna in theory is a Huygens source with cardiod antenna diagram and directivity 4.77 dBi. It may be two dipoles electrical and magnetic. Magnetic dipole may be a loop or slot.